Charged residue changes in the carboxy‐terminus of α‐tropomyosin alter mouse cardiac muscle contractility

Striated muscle tropomyosin (TM) is an essential thin filament protein that is sterically and allosterically involved in calcium-mediated cardiac contraction. We have previously shown that overexpressing the β-TM isoform in mouse hearts leads to physiological changes in myocardial relaxation and Ca2+ handling of myofilaments. Two important charge differences in β-TM compared to α-TM are the exchange of serine and histidine at positions 229 and 276 with glutamic acid and asparagine, respectively, imparting a more negative charge to β-TM relative to α-TM. Our hypothesis is that the net charge at specific sites on TM might be a major determinant of its role in modulating cardiac muscle performance and in regulating Ca2+ sensitivity of the myofilaments. To address this, we generated transgenic (TG) double mutation mouse lines (α-TM DM) expressing mutated α-TM at the two residues that differ between α- and β-TM (Ser229Glu + His276Asn). Molecular analyses show 60–88% of the native TM is replaced with α-TM DM in the different TG lines. Work-performing heart analyses show that α-TM DM mouse hearts exhibit decreased rates of pressure development and relaxation (+dP/dt and –dP/dt). Skinned myofibre preparations from the TG hearts indicate a decrease in calcium sensitivity of steady state force. Protein modelling studies show that these two charge alterations in α-TM cause a change in the surface charges of the molecule. Our results provide the first evidence that charge changes at the carboxy-terminal of α-TM alter the functional characteristics of the heart at both the whole organ and myofilament levels.